Various systems for use in providing hypodermic injection of medication without piercing the skin with a needle are known in the art. Such systems can include hypodermic injection devices powered by pressurized gas.
One such needleless hypodermic injection device is disclosed in U.S. Pat. No. 4,680,027 which issued on July 14, 1987 to James S. Parsons and Jack S. Gasaway. This device includes a disposable syringe which can be inserted into a power supply mechanism. The syringe has a cavity for holding liquid medication and an aperture on one end through which liquid medication can flow into and out from the cavity. A plunger is in the cavity for drawing liquid medication into the cavity and for forcing the liquid medication out from the cavity. The syringe has a hollow, tubular needle removably attached to it with the needle in alignment with the aperture in the end of the syringe. The needle is insertable into a container of liquid medication so that the syringe can be filled by pulling on the plunger in a conventional manner. The needle is removable from the syringe after the liquid medication has been drawn into the syringe so that the aperture in the end of the syringe can be placed in direct contact with the skin.
The power supply mechanism has a trigger which, when pulled, unseals a container of compressed gas to release the gas. The released gas provides a force to move the syringe plunger toward the aperture so that the liquid medication is ejected through the aperture with sufficient force to penetrate the skin. The pressure applied to the liquid medication can be varied by changing the spring constant of a spring which might be used, or by adjusting a needle valve to reduce the flow of gas to the piston.
The above-described power supply mechanism includes safety interlocks, one of which prevents securing the syringe within the power supply mechanism until the needle is removed. A second interlock prevents movement of the trigger so that the gas container cannot be opened unless the syringe is fully secured in the power supply mechanism. Both interlocks are relatively complex mechanical systems.
It is desirable to provide a needleless hypodermic injection device with a relatively simple, easy-to-use, and reliable system for varying the gas pressure applied to the liquid medication so that the medication can be ejected at a desired velocity. Such reliable ejection velocity control can be important, for example, because discomfort is minimized when the minimum velocity required to appropriately penetrate the skin is used. Minimum required velocities can be different for different injection locations, and more or less gas pressure can be required to attain the minimum required velocity when medications of different viscosities are injected.
There is a need for a needleless hypodermic injection device which has an enhanced simplicity of design, including a simple design of the interlock mechanisms, but yet which provides for improved user comfort by incorporating a reliable system for adjusting medication ejection velocity.